1. Field of the Invention
The present invention relates to a color sensor. Moreover, the present invention relates to an electronic device having the color sensor.
2. Description of the Related Art
A number of optical sensors that convert the intensity of light into electrical signals and output have been known and have sensitivity to electromagnetic waves ranging from ultra violet rays to infrared rays. A number of optical sensors are used for adjustment of illuminance in accordance with living environment of humans beings and for devices which require on/off control.
In some display devices, ambient brightness of the display device is detected so that display luminance is adjusted. This is because unnecessary electric power of the display device can be reduced by increasing visibility through detecting ambient brightness by an optical sensor and obtaining appropriate display luminance. For example, examples of display devices which have an optical sensor for adjusting luminance include a mobile phone and a computer with a display portion. In addition, as well as the ambient brightness of the display device, luminance of the backlight of a display device, in particular, a liquid crystal display device is also detected by an optical sensor to adjust luminance of a display screen.
The optical sensor includes a photoelectric conversion element such as a photo diode in a light sensing portion and can detect illuminance based on the amount of current which flows to the photoelectric conversion element. Patent Document 1 discloses a structure in which photo current generated by the photoelectric conversion element is made to flow to a diode element in order to obtain an output signal as a voltage value which has been subjected to logarithmic compression so that a dynamic range of the illuminance of the optical sensor is obtained. Note that the logarithmic compression is to obtain a current value or a voltage value output with the illuminance of light which enters the photoelectric conversion element, that is, the value of the photo current used as a variable, so that the output current value or voltage value is used as a logarithmic function. Moreover, Patent Document 1 discloses a structure in which an analog value obtained on the basis of current which flows from a photo diode in accordance with the amount of incident light is converted into a digital value to be used as an output signal.
Further, Patent Document 2 discloses a color sensor in which a plurality of optical sensors is provided over a substrate to receive light with different wavelengths.    [Patent Document 1] Japanese Published Patent Application No. 2008-124568    [Patent Document 2] Japanese Published Patent Application No. 2006-135320